A horizontal well, inter alia, considerably increases the productive length and therefore the contact surface with the geological formation in which gas and/or oil is present in source rock.
In such a horizontal configuration, it is technically difficult to case and cement the annular space between the pipe and the inner wall of the well in a horizontal position. This cementing technique, used in the majority of vertical or slightly deviated wells, provides a seal between different geological zones.
The exploitation of horizontal wells, whether for stimulation or flow control, requires some zones to be isolated in the rock formation itself.
A pipe is run into the well with isolation devices at its periphery, spaced out in predetermined fashion.
The term “zonal isolation packers” is used for these devices. Between these isolation devices the pipe often has ports open or closed on demand, which enable communication between the pipe and the isolated zone of the well.
In this horizontal completion environment, hydraulic fracturing (also called “fracking”) is a technique for cracking of the rock in which the pipe is set horizontally.
Fracking is carried out by injection of a liquid under pressure. This technique enables extraction of oil or gas contained in highly compact and impermeable rocks.
The injected liquid generally comprises 99% water mixed especially with sand or ceramic microballs. The rock fractures under the effect of pressure and solid elements penetrate inside fissures and keep them open when the pressure drops so that gas or oil can flow through the resulting breaches.
These days fracking is mostly carried out by using an assembly of pipes such as described above. The zones are fractured one by one so that the quantity of fluid injected can be controlled. The fluid is indeed injected in limited volumes that are spread along the well. Pressures up to 1000 bar (15000 psi) can be reached.
A key element of these fracking completions is located in the isolation and sealing device. It has to ensure perfect sealing between the zones to guarantee the quality and safety of fracking.
Indeed, if sealing not ensured, a zone could be fractured several times, creating an excessively large fracture and reaching unplanned geological zones.
During these fracking operations, isolation devices are subjected to high internal, external and differential pressures. Also, the injected fluids often have a lower temperature than that of the well, subjecting isolation devices to variations in temperature.
Several types of isolation devices are currently being used.
Hydraulic-set isolation devices “Hydraulic Packers” which utilise hydraulic pressure to compress a rubber ring via one or more pistons are being used.
This rubber ring expands radially and comes into contact with the borehole.
The U.S. Pat. No. 7,571,765 is a typical example of this type of hydraulic-set isolation device.
It is clear that when used this type of device does not properly seal a well having an ovalised cross-section.
Also, a fracture of the rock can be initiated at the packer level due to high contact pressure. Hydraulic isolation devices are also sensitive to temperature variations.
Other types of devices can be used.
In this way, mechanical isolation devices “mechanical packers” have a working principle close to that of hydraulic isolation devices, the only difference is that the compression of the rubber ring is carried out by an external tool.
Also, inflatable isolation devices (in English “inflatable packers”) comprise an elastic membrane inflated by injection of liquid under pressure. After activation, the pressure is maintained in the sealing device by check valve systems.
Isolation devices based on swellable elastomer (in English “swellable packers”) are composed of an elastomer which swells when placed in contact with a type of fluid (oil, water, etc.) according to formulations.
Activation of these devices is initiated by contact with fluid. It is therefore understood that diameter increase must be relatively slow so as to avoid blockage of the completion during the run in hole. As a consequence, it sometimes takes several weeks to achieve the isolation of the zone.
Other types of isolation devices are those known as “expandable” (in English “expandable packers” or “metal packers”) and comprise an expandable metallic sleeve which is deformed by application of liquid under pressure (see the article SPE 22 858 “Analytical and Experimental Evaluation of Expanded Metal Packers For Well Completion Services ” (D. S. Dreesen et al.-1991), U.S. Pat. Nos. 6,640,893 and 7,306,033).
Expandable isolation devices made of metal usually comprise a ductile metallic sleeve attached and sealed at its ends to the surface of a pipe. The interior of the pipe, on the one hand, and the ring defined by the external surface of the pipe and the inner surface of the expandable sleeve, on the other hand, communicate with each other. The metallic sleeve is expanded radially towards the exterior until it makes contact with the borehole, by increasing the pressure in the pipe to create an annular barrier.
Contrary to other isolation devices, sealing is not based on elastomer means only, whereof the efficiency over time and under severe conditions is uncertain. Also, fracking often makes use of fluids at external ambient temperature whereas isolation devices are brought to the temperature of the well.
Expandable metal sleeves are less sensitive to temperature variations and more particularly to thermal contraction. The value of the coefficient of thermal expansion of the metal is lower than that of elastomer.
These expandable metal isolation devices therefore combine the advantages of devices explained earlier. First, as isolation devices based on inflatable elastomer, their design is simple and inexpensive and also they can be activated on demand as hydraulic isolation devices, soon after the completion has been run in the well.
Purely by way of illustration FIG. 1 illustrates a portion of pipe capable of being run in a well. This portion of pipe illustrated here is provided with two isolation devices 2 between which extends a portion of pipe 1 which presents a set of through openings 3.
This pipe 1 is illustrated again in the bottom part of the figure, the isolation devices 2 set in an expanded position.
The arrow v represents the circulation of fluid inside the pipe for fracking, that is, from upstream to downstream.
FIG. 2 is a simplified sectional view of the pipe such as that in FIG. 1, which extends into a previously prepared well.
The aim of the description of this figure is simply to explain how pipes provided with such zonal isolation devices has been used to date.
A well A whereof the wall is referenced A1has previously been drilled in the ground S.
Pipe 1 which is illustrated partially here has been set in place inside this well.
Along its wall, this pipe has, at regular intervals, isolation devices 2. In this case, just two devices 2 designated N and N-1 are illustrated by way of simplification.
In practice, there is a larger and substantial number of such devices along the pipe. As is known, each device is constituted by a tubular metallic sleeve 20 whereof the opposite ends are connected directly or indirectly to the external face of the pipe by reinforcing rings or skirts 21.
Pressure P0 prevails in the well.
Initially, the metallic sleeves 20, not deformed, extend substantially in the extension of the rings 21.
The distal end of the pipe preferably comprises a port, not illustrated here, which is initially open during the descent of the pipe into the well so as to allow circulation of fluid from upstream to downstream at pressure P0. This port is preferably closed by means of a ball which is placed in and blocks this port, increasing the pressure in the pipe is then possible.
A first fluid under pressure P1 greater than P0 is then sent inside the pipe. The fluid circulates through openings 10 arranged in front of the sleeves 20 along the entire pipe so as to expand the metallic sleeves and take the position of FIG. 2 in which their intermediate central part is in contact with the wall A1 of the well.
Of course, the material of the sleeve and the pressure are selected so that the metal deforms beyond its elastic limit.
A device, not illustrated, frees up an opening located at the distal end of the pipe when the pressure P1 is slightly raised. The pressure at the level of the opening goes from P1 to P0 and circulation is then possible in the pipe from upstream to downstream of the well.
Next, another ball 5 is launched inside the pipe and lands in a sliding seat 4 located substantially mid-distance between the two isolation devices N and N-1.
Originally, the seat 4 is located just opposite the abovementioned openings 3 and seals them. Under the effect of displacement of the ball, the seat 4 is closed and shifts, freeing up the openings 3. A fracking fluid under very high pressure is then injected inside the pipe 1.
This fluid, under pressure P2, is introduced in the device N as well as in the annular space B which separates the devices N and N-1.
However, the prevailing pressure inside the device N-1 returns to the initial pressure of the well, that is, to the pressure P0.
In these conditions, the difference in pressure which exists between the annular space B and the device N-1 exposes the sleeve 2 of the device N to high stresses which in some places leads it to partially collapse. It is understood that this constitutes a source of leaks, meaning that the zone B to be fracked is no longer fluid or gas tight.
Systems have been added to this kind of devices to withstand collapse. An example is given in document WO 2011/042492. Another option is to use this pressure difference by way of valves to maintain internal pressure in the device after expansion or to “capture” this pressure difference (see U.S. Pat. No. 7,591,321, U.S. Publication No. 2006/004801 and U.S. Publication No. 2011/0266004). Yet, all these solutions mean greater complexity of the material and risk of malfunctioning.
From EP-A-1 624 152 is known a device in which each sleeve of the pipe is equipped with a “skin” which extends only along a part of said sleeve. Between the sleeve and the skin is present a sealant material.